Pressure responsive pump control



PRESSURE RESPONSIVE PUM? CONTROL Filed Jan. 14, 1965 2 Sheets-Sheet l INVENTOR Fwy/mp. /'7/ 767/614.

ATTORNEYS Feb. 7, 1967 R. D. MITCHELL 3,302,574

PRESSURE RESPONSIVE PUMP CONTROL Filed Jan. 14, 1965 2 Sheets-Sheet 2 ATTORNEY United States Patent 3,302,574 PRESSURE RESPONSEVE PUMP CONTROL Ryan D. Mitchell, Thomasville, Ga, assignor to Davco Manufacturing Corporation, Thomasville, (3a., a corporation of Georgia Filed Jan. 14, 1965. Ser. No. 425,414 6 Claims. ((11. 103-25) This invention relates to fluid pumping system of the type in which pressure signals derived from the fluid pressure in the system are employed to switch a pump on and off according to a predetermined pattern of Operation. More in particular, it relates to the problem of overcoming undesirable effects on the pump control system resulting from high pressure differentials occurring at pump start-up.

In one preferred form, the invention relates to a simplified and effective pressure sensing arrangement for use with a liquid sump and associated pump, the pump being employed to withdraw liquid from the sump when the liquid level rises to a predetermined height. A specific example of a system of this kind is a pumping station in a sewage handling system. The station conventionally includes a wet well which is usually in the form of a relatively deep tank or the like serving as a collector for raw sewage entering the tank by gravity or from another pumping station. The sewage is pumped at intewals from the wet well to a tank at a higher location or to a treatment plant.

One conventional pump control device for a wet well includes a bubbler tube projecting down into the sewage in the tank and provided with a supply of compressed air, as from a motor-driven compressor. The lower end of the tube is open so as to permit air to bubble out into the sewage continuously. As the level of the liquid in the tank rises, the pressure at the lower end of the tube, and consequently in the air within the tube, rises. A pressure sensitive switch associated with the tube at a location above the sewage is set to turn on the pump at a predetermined high pressure. Similarly, the pressure in the tube falls with a drop in liquid level in the tank and the switch is set to turn otr" the pump at a predetermined low pressure. Other level sensing devices, such as floats, are also employed in well pump-out systems.

It is one object of the present invention to provide a pump control system which eliminates a bubbler tube or any other pressure sensing element in a sump. Broadly, this is accomplished by tapping a pressure sensitive device, including a switch, into the pump suction line at a location outside the sump. Since the pump suction line is open below the level of the liquid, changes in the liquid level in the sump create measurable changes in pressure in the suction line. In the case of sewage wet wells, where it is customary to provide two pumps which are used alternately, a switch is tapped into each line at the same level so that an identical signal is received by each switch.

A more important object is to provide a pressure responsive pump control system having pressure transmitting means which delays the effect on the pressuresensitive elements of temporary, relatively large pressure differentials occurring in the fluid at pump start-up there by rendering the control system independent of the startup pressure diiferential.

It is known that the magnitude of pump suction and pump discharge pressure are greater immediately after pump start-up than when the pump is running normally. This effect has been recognized as resulting from the inertia of the fluid in the suction and discharge lines of the pump. In pump control systems which derive their on-off signals from a device which measures the absolute pressure of the fluid, the start-up pressure efiect is often disadvantageous because it causes a sudden, erroneous pressure signal to be transmitted to the pump. For example, in known systems for pumping water from a reservoir or the like to an elevated storage tank, the startup inertia pulse in the pump discharge line, if transmitted directly to the pressure sensitive elements connected to the discharge line, would tend to turn off the pump. This would occur because the pressure responsive part of the control system is set to turn oft" the pump when the ressure in the discharge line reaches a predetermined maximum figure corresponding to the desired maximum liquid level in the elevated tank. The result is that the pump would be turned off and on a large number of times before normal operation is arrived at. This diificulty is sometimes referred to as short cycling of the pump.

The start-up pressure change in a pumping system has generally been regarded in the past as being a series of closely spaced, high pressure pulses in the pump discharge line. Various proposals based on the use of snubbing devices have been made, the theory being that if the amplitude of the pulses is reduced sufficiently they will not actuate the pressure sensitive pump switch. Snubbers, generally, are merely a restricted orifice in the line leading from the pump discharge line to the pressure sensitive element. Broadly, the term snubber would also include the use of an air cushion in the same location. In either case the function of the snubber is to act as a shock absorber to reduce, at least partially, the magnitude of the pressure pulses.

The present invention presents a different approach to the problem by providing a simply constructed pneumatic delay device which delays the transmittance of startup pressure changes to the pressure responsive elements for a period of time at least equal to the period of the start-up pressure change. Specifically, the invention contemplates the use of the delay device between a pressure tap on a pump suction line and a pressure responsive control element in a pump-out system, such as a sewage pumping station. This preferred arrangement has the additional advantage, already referred to, of eliminating level sensing devices in the wet well and the auxiliary equipment necessitated by such devices. Further, the delay device may be easily tapped into a plurality of pump-out lines at a location to effect an identical control signal on each of several pumps.

Specifically, the pneumatic delay device of the present invention comprises a restricted passage communicating at opposite ends with air-filled chambers of substantial volume. In one preferred construction the device consists of an air-filled container, or surge tank, divided into two generally equal chambers by a transverse plate or the like having a small orifice therethrough. One chamber communicates with a pressure tap in the suction line of the pump and the other chamber communicates with the pressure responsive element of the control system, such as a conventional pressure-sensitive switch. The delay function of the tank is effected by cooperative action of the two chambers together with the orifice. The purpose of the orifice is to permit air to pass from one chamber to the other only at a slow volumetric rate with the result that a large pressure change in the chamber connected to the pressure tap is transmitted only slowly to the other chamber. The chambers are necessary in order to provide a supply of air.

The restricted passage, or orifice, and the air-filled chambers of the present invention should not be confused with conventional amplitude-dampers, or snubbers. The latter reduce the magnitude of a sudden pressure change but they do not reduce, to any appreciable extent, the time between the occurrence of the change and the arrival of the change at the pressure responsive elements. This is particularly true where the snubbers consist of an ori- 35 iice in a liquid filled line. The presence of the restricted passage, or orifice, in the delay device of the present invention will, of course, inherently produce some snubbing action, but this effect does not constitute part of the invention.

Other objects and advantages will be apparent from a reading of the following detailed description taken in conjunction with the drawings in which:

FIGURE 1 is a side elevational view of a pumping .system embodying the principles of the present invention;

FIGURE 2 is a top plan view of the system of FIG- URE 1;

FIGURE 3 is a sectional view, on an enlarged scale, taken on the line 33 of FIGURE 1 and showing the surge tank and associated elements rotated 90 for simplicity; and

FIGURE 4 is a side elevational view of a modified form of the pumping system of FIGURE 1. Referring to FIGURES 1 and 2 there is shown a sewage pumping station I9 which embodies the pump control features of the present invention. The pumping station includes a below-ground concrete wet well 12 adapted to receive raw sewage through an inlet pipe The discharge end of the pipe I4 is provided with a coarse strainer 16 to remove and retain heavy solid objects such as rocks and pieces of metal. The strainer I6 can be disconnected from the pipe 14 and removed from the well through a door 18 in the cover 24) of the well. The bottom of the well is inclined at 22 so that sludge which tends to settle out of the sewage is directed toward the lower ends of a pair of vertical pump-out pipes, 24, 26. A typical wet well may he 6 feet in diameter by 20 feet deep.

Each pump-out pipe 24, 26 is supported from the wall of the wet well by means of suitable brackets (not shown) and extends upwardly from near the bottom of the well through the well cover 2d. The upper end of each pipe 24, 26 is connected to a short horizontal pipe section 23, 3t? which leads to the inlet of a separate pump 32, The pumps 32, 34 are of a self-priming centrifugal type, and each is driven by a separate electric motor 36, 38. The pump outlets are connected to separate pipes 4t 42 which discharge into a main discharge line leading to another wet well or to a sewage treatment plant. The pumps and motors are supported in any suitabic manner as by means of a cement slab at ground level. In practice, the pumps and motors will be pro vided with conventional auxiliary equipment (not shown) such as an instrument panel and manual controls (not shown), and will be housed within a shed (not shown) to protect the equipment from the weather. In normal operation only one pump operates at a time, the other being a standby pump for use when the first pump is shut down for maintenance. The pumps are operated alternately to even the wear on them.

According to the present invention, and referring especially to FIGURE 3, the pump motors 36, 38 are turned on and off by means of a control assembly 43 which includes a pair of conventional pressure sensitive switches 72, 74 and an air-filled surge tank 54 connected together in a particular manner to be described in detail hereinafter. As shown, the control assembly 48 is tapped into the horizontal pipe sections at 56 and 58 by means of a small diameter tap line 6!? which is connected between the horizontal pipe sections 28 and 39. The location of the connections 56 and 58 is not critical except that they should be made at the same level on each suc tion line and they should be made in a position such that liquid will not tend to pass into or become trapped in the control assembly Connection of the tap line it through the upper surfaces of the horizontal pipe sections 28, 39, as shown, satisfies both these conditions.

Intermediate the ends of the tap line 60 and extending upwardly from the top surface thereof is a flexible line 62 which connects with the bottom wall of the surge tank 54. The latter consists of a cylindrical tank divided into equi-sized upper and lower chambers 64, 66 by a partition 68 which is sealed .to the inner wall of the tank. The partition 6% has a small orifice 70 therethrough which restricts the rapid passage of air from one chamber to the other upon the occurrence of a pressure diiferential across the partition 68.

The two conventional pressure-sensitive switches 72, 74 are located above the surge tank 54 and are connected to receive pressure signals from the upper chamber 64. As shown, a short vertical line extends from the upper wall of the upper chamber 64 to a connector 76 from which a separate branch line 78, 843 extends horizontally and then upwardly to each switch. If desired, the connector '76 may also carry a pressure gauge 82 for giving a visual reading of the pressure in the pump suction lines. The switches 72, 74- are designed and adjusted to produce an electrical pump-on signal at a predetermined high pres sure and a pump-off signal at a predetermined low pressure. Each switch is associated with only one pump, and there is provided a control circuit (not shown) which interconnects the switches and motors. The circuit is of a conventional type which activates only one of the pumps upon receiving an on signal from the sensing device, deactivates that pump upon receiving an off signal and activates the other pump upon receiving the next on signal.

If desired, the branch lines 78, 8t connecting with the pressure switches 72, '74 may have conventional snubbers (not shown) incorporated therein for further reducing the aptitude of violent, short-duration pressure changes in the control assembly 4-8. In this regard it should be understood that snubbers would not be the same as or analogous to the surge tank 54 in either structure or function. Snubbers would reduce the magnitude of a sudden large pressure change which is of short duration, whereas the surge tank 54' creates a delay in transmitting a large pressure change of longer duration. This difference between the two structures will be fully apparent from the description of the operation of the control assembly which is given later.

Referring to FIGURE 4 there is shown another sewage pumping station Til which differs from the equipment shown in FIGURES 13 primarily in that the pumps and control assembly are located below the normal sewage level in the well. As shown, the station It) includes a below-ground, cylindrical concrete tank T2, or wet well, which receives raw sewage from an inlet pipe 14'. A separate below-ground chamber 86 houses two side-byside motor-pump units, only one of which is seen in FIG- URE 4 at 8%, 9d. The pumps are of a conventional centrifugal type and are mounted below their respective notors with their axes extending vertically. The inlet of the pump 90 is connected to the bottom of the wet well 12 by a horizontal suction line 92, and the other pump is similarly connected. The outlets of both pumps are connected to a main discharge line 94 which extends upwardly through the roof of the chamber 86.

The suction line 92 and the other suction line (not shown) are provided with a pump control assembly 48 identical with that shown in FIGURE 3. As shown, the assembly 48' is located in the chamber 36 and consists of a horizontal tap line oil, a surge tank 54, two pressure switches 72, '74, and a pressure gauge 82. As in the previous embodiment the assembly 48' is employed with a control circuit (not shown) for operating alternate pumps in accordance with the electrical signals from the switches '72, '74-.

Referring again to FIGURES 1-3 and specifically to the operation of the system shown therein, the wet well Iii will receive raw sewage intermittently or continuously from the inlet pipe 14-. As the level of sewage rises in the well, the static pressure head in the two pump-out pipes 24, 26 will rise due to the head of water above the lower ends of the latter. This pressure will occur because the pipes 24, 26 are constructed with airtight joints, and because the self-priming pumps effectively close the upper ends of the pipes. The pressure rise in the pipes 24, 26 is transmitted through the horizontal tap line 60 to the air which is permanently trapped in the flexible line 62 of the control assembly 48. Since the rise in sewage level 106 is gradual, the pressure rise in the flexible line 62 is gradual and is readily transmitted by a flowing of air through the orifice '70 of the surge tank 54 to the switches 72, 74. The latter are set to generate a pumpon signal when the static pressure head in the assembly 48 reaches a value which corresponds to the maximum desired level 100 of sewage in the well. Therefore, when the sewage level 100 reaches the predetermined maximum, one of the switches, for example the righthand switch 72, together with the control circuit (not shown) turns on the righthand pump 32.

As the sewage level 100 decreases toward a predetermined low level which is selected at some point above the lower ends of the pump-out pipes 24, 26, the air pressure in the control assembly 48 falls as a result of the decreasing pressure head of liquid acting on the liquid in the pump-out pipes 24, 26. The pressure in the branch lines 78, 80 will substantially coincide with the ressure in the lower chamber 66 because the gradual change in pressure will not be affected by the orifice. When the sewage level 1% reaches the predetermined low level, the same switch 72 generates pump-off signal to stop the pump 32. Upon filling of the well 12 again to the pre determined high level, the control circuit (not shown) passes an on signal from the other switch 74 to the other pump 34. Since both switches 72, 74 are tapped into the pump-out pipes 24, 26 at the same height above the sewage level 100, the on and off signals will be generated at the same high and low sewage levels in the well regardless of which switchpump combination is operating. As indicated above, and still referring to FIG- URES 1-3, a temporary and severe drop in pressure occurs at the inlet of either pump when it is started, and this drop will be reflected in a similar pressure drop within the tap line 60 and the flexible line 62 of the control assembly 48. The duration of the low pressure is of the order of a few seconds to a minute or more and its magnitude, in many cases, would be so small that it would cause the switches 72, '74 to produce a pump-off signal. The surge tank 54 between the pump-out pipes 24, 26 and the switches 72, 74 delays the transmission of this low pressure surge to the switches 72, 74 for a period at least equal to the duration of the surge. Therefore, the surge tank 54 prevents the operating switch from turning off its pump during the period immediately following pump startup.

The surge tank 54 effects a delay in transmitting the low pressure surge by virtue of its two chambers 64, 66 separated by the orifice 70. Assuming that the lower chamber 66 is full of air, the low pressure surge in the tap line 60 and in the flexible line 62 will be initially cushioned and reduced in magnitude by the relatively large volume of air in the lower chamber 66. The pressure in the lower chamber 66 will, of course, fall, but this drop is not rapidly transmitted to the upper chamber 64, because air passes through the orifice 70 into the lower chamber 66 only at a relatively slow tvolumetric rate. Further, the relatively large volume of air in the upper chamber 64 serves to cushion the pressure drop which does take place across the orifice. Accordingly, the low pressure surge existing in the suction line of the pump does not effect. or even reach, the pressure switches 72, 74. By the time the pressure drop would have completely passed through the orifice 70, the pump has begun to pump normally and there exists in the tap line 60 a suction pressure which varies according to the sewage level 100 in the well. The absolute pressure in the line is dependent, of course, on sewage level, pump suction and friction but the last two variables are substantially constant for a given system.

The lower chamber 66 of the surge tank 54 serves yet another function in that it will collect a substantial volume of liquid which, if allowed to pass through the orifice, would destroy the critical delay effect described above. From time to time water may back up into the line 62 which is connected to the bottom wall of the lower chamber 66. Since this line 66 is of small volume, even a relatively small amount of water would pass directly to the orifice 70 were it not for the presence of the lower chamber 66. If the orifice '70 were submerged in water, the pressure delay characteristics of the control assembly would be entirely different and would render the device unfit for .its intended purpose. It will be understood that the surge tank 54 will always be disposed above the pump suction lines 26, 28 or that some other precaution will be taken to prevent liquid from entering the tank 54.

During normal operation of either of the pumps 32, 34 a small amount of liquid will flow through the tap line 60 from one pump-out pipe into the pump-out pipe to which suction is being applied. This flow will be accompanied by friction losses which will afiect the static pressure in the tap line 60. In order that the effect of this pressure be equal regardless of the direction of flow in the tap line 60, it is important that the connection of the lower chamber 66 to the tap line 60 be made at a neutral location. In most cases the connection will be made in the middle of the tap line 60, as shown.

It is desired, of course, that the control assembly 48 and its circuit will effect the operation of one or the other of the pumps at the same sewage levels in the Well regardless of which pump is being used. Desirably, identical pressures will the transmitted to the switches 72, 74 so that the latter may be of the same construction and setting. In the embodiment of FIGURES 1-3 it is assumed that the pumps 32, 34 and pump-out lines 26, 28 have identical hydraulic characteristics. This permits .a symmetrical arrangement in which the tap line 60 is horizontal and is connected to the pump-out pipes 26, 23 at a convenient identical height. However, if the characteristics of the pumps 32, 34 or pumpout pipes 26, 28 are not the same, equal pressure signals can be produced by adjusting the locations at which the tap line 60 is connected to the pump-out pipes and/or adjusting the location of the connection between the lower chamber 66 and the tap line 60. In most cases it is only necessary to locate the latter connection closer to one end of the tap line 60 than to the other end. Accordingly, the use of a tap line, as shown, is advantageous because it permits easy adjustment of the pressure signals to a common value.

The operation of the system shown in FIGURE 4 is substantially the same as the operation of the system of FIGURES 1-3 and requires no detailed description. Since the pump 88 and its twin (not shown) are located below the sewage level 10 9' in the wet well 12' there will be a positive pressure vhead on the pump inlets when no pump is running, rather than a negative head as in the FIGURE 1 system. Accordingly, the absolute pressures within the control assembly 48 will not vary between the same limits as in the FIGURE 1 system, and different set-tings for the switches 72' and 74 will be required.

It will be understood from the above description of the control assembly that the size of the chambers 64 and 66 in the surge tank 54 and the size of the orifice 70 or similar restriction between the chambers will depend primarily upon the hydraulic characteristics of the pump ing system. That is, the size of the tank 54 will depend on the suction characteristics of the pumps 32 and 34, on the volume of the lines connecting the surge tank to the switches 72 and 74 and on the volume of the lines connecting the surge tank 54 to the pressure taps on the pump suction lines. It is therefore not practical,

in defining the invention, to specify a precise size for the chambers or orifice.

Thus, it will be seen that the present invention provides a pump control assembly which is simple in construction and which does not require any level sensing element immersed in the liquid or even disposed inside the wet well. The assembly, in incorporating a pressure delay feature, also overcomes the effect of the startup pressure surge which would otherwise render the switches inoperative under certain conditions.

While preferred embodiments of the invention have been described and illustrated, it is contemplated that modifications can be made by one skilled in the art without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

1. In combination with a sump or the like, a pump out system for maintaining the surface of liquid in said sump between predetermined levels, said system comprising: first and second motor driven pumps disposed adjacent said sump, each pump having an inlet and an outlet; circuit means interconnecting said pumps for operating said pumps alternately; a separate pump-out pipe connected to each pump inlet and communicating with the liquid in said sump; a pressure tap conduit extending between said pump-out pipes at a location outside said sump, said conduit passing liquid in the direction determined by whichever pump is operating; first and second pressure sensitive elements for generating a control signal for the drive motor of said first and second pumps, respectively; and gas-filled conduit means extending from said pressure sensitive elements to a common junction with said pressure tap conduit, said junction being disposed intermediate the ends of said pressure tap line, whereby the locations of the connections between the pressure tap line and the pump-out pipes and between the pressure tap line and the gas-filled conduit means may be selected so as to compensate for differences in the hydraulic characteristics of the sets of pump and pump-out pipes and to thereby provide equal pressure signals to said pressure sensitive elements regardless of which pump is operating.

2. A liquid pumping system comprising a motor driven pump having separate inlet and outlet conduits, at least one of said conduits having an open end submerged in a body of liquid to be pumped and being sealed at its other end by the pump when the latter is not operating so that said one conduit remains full of liquid whereby gradual changes in the static pressure head acting on the submerged end of said one conduit as a result of changes in the level of the body of liquid are transmitted through the liquid in said one conduit, said one conduit having pressure-sensitive switch means associated therewith for generating pump-on and pump-off signals in accordance with different predetermined levels of the body of liquid, time delay means between said one conduit and said switch means for preventing appreciable transmittance to said switch means of the temporary pressure surge which occurs upon starting or stopping of said pump thereby preventing the generation of a false pump signal, said time delay means including first and second gascontaining enclosures interconnected by gas-flow restricting means for passing gas from one enclosure to the other at low flow rate, said first enclosure being in communication with said one conduit whereby the gas pressure in said first enclosure follows the pressure in said one conduit, said second enclosure being in communication with said switch means whereby the pressure in said second enclosure acts on said switch means, said enclosures being of such large volume and the gas flow through said restricting means being of such small magnitude that the magnitude of the temporary pressure surge which occurs in said first enclosure upon starting or stopping said pump decreases before any appreciable corresponding pressure change occurs in said second enclosure as a result of the low rate of gas flow through said restricting means and the accompanying small pressure change in said second enclosure whereby said enclosures and restricting means act as a time delay for transmitting pressure surges to said switch means but do not affect the transmittal of slow pressure changes resulting from changes in the level of the body of liquid.

3. Apparatus as in claim 2 wherein said delay means includes a surge tank having an interior partition dividing said tank into two portions which constitute said two enclosures, said partition having at least one aperture therein which constitutes said gas flow restricting means.

4. Apparatus as in claim 2 wherein said one conduit is the pump suction conduit and wherein said pressure sensitive switch is adapted to generate a pump-on signal at a predetermined low level of the body of liquid and a pump-cit signal at a predetermined high level of the body of liquid.

5. Apparatus as in claim 2 including a second pump and a second pressure sensitive switch corresponding thereto, said second pump having a flow conduit corresponding to said one conduit with which said first gascontaining enclosure is in communication, said first enclosure being in communication with said flow conduit and said second gas-containing enclosure being in communication with said second switch whereby pressure changes transmitted by said time delay means are transmitted equally to each of said switches.

6. Apparatus as in claim 5 including a tap line extending between said one conduit and said flow conduit and including a single pressure conducting line extending from said tap line intermediate the ends of the latter to said first enclosure, said pressure conducting line defining the only communication between said first enclosure and said conduits.

References Cited by the Examiner UNITED STATES PATENTS 1,670,944 5/1928 Vawter 103-25 1,767,100 6/1930 Tamehill 10325 1,842,295 1/1932 Schurle s l0325 2,526,646 10/1950 Ericson l03ll 2,765,743 10/1956 Hollinshead 10325 2,767,277 10/1956 Wirth 10325 X 2,812,110 11/1957 Romanowski 10311 2,910,003 10/1959 Kaatz 10325 2,953,659 9/1960 Edwards l03-25 X 3,093,083 6/1963 Nielsen 10325 3,112,760 12/1963 Budd l0325 X MARK NEWMAN, Primary Examiner.

r SAMUEL LEVINE, Examiner. 0 W. L. FREEH, Assistant Examiner. 

1. IN COMBINATION WITH A SUMP OR THE LIKE, A PUMPOUT SYSTEM FOR MAINTAINING THE SURFACE OF LIQUID IN SAID SUMP BETWEEN PREDETERMINED LEVELS, SAID SYSTEM COMPRISING: FIRST AND SECOND MOTOR DRIVING PUMPS DISPOSED ADJACENT SAID SUMP, EACH PUMP HAVING AN INLET AND AN OUTLET; CIRCUIT MEANS INTERCONNECTING SAID PUMPS FOR OPERATIN SAID PUMPS ALTERNATELY; A SEPARATE PUMP-OUT PIPE CONNECTED TO EACH PUMP INLET AND COMMUNICATING WITH THE LIQUID IN SAID SUMP; A PRESSURE TAP CONDUIT EXTENING BETWEEN SAID PUMP-OUT PIPES AT A LOCATION OUTSIDE SAID SUMP, SAID CONDUIT PASSING LIQUID IN THE DIRECTION DETERMINED BY WHICHEVER PUMP IS OPERATING; FIRST AND SECOND PRESSURE SENSITIVE ELEMENTS FOR GENERATING A CONTROL SIGNAL FOR THE DRIVE MOTOR OF SAID FIRST AND SECOND PUMPS, RESPECTIVELY; AND GAS-FILLED CONDUIT MEANS EXTENDING FROM SAID PRESSURE SENSITIVE ELEMENTS TO A COMMON JUNCTION WITH SAID PRESSURE TAP CONDUIT, SAID JUNCTION BEING DISPOSED INTERMEDIATE THE ENDS OF SAID PRESSURE TAP LINE, WHEREBY THE LOCATIONS OF THE CONNECTIONS BETWEEN THE PRESSURE TAP LINE AND THE PUMP-OUT PIPES AND BETWEEN THE PRESSURE TAP LINE AND THE GAS-FILLED CONDUIT MEANS MAY BE SELECTED SO AS TO COMPENSATE FOR DIFFERENCES IN THE HYDRAULIC CHARACTERISTICS OF THE SETS OF PUMP AND PUMP-OUT PIPES AND TO THEREBY PROVIDE EQUAL PRESSURE SIGNALS TO SAID PRESSURE SENSITIVE ELEMENTS REGARDLESS OF WHICH PUMP IS OPERATING. 